Connecting rod failure leads to forced landing

The commercial pilot and observer were conducting an aerial observation flight of a pipeline in the Cessna 172S.

While maneuvering at a low altitude toward an airport to refuel, the pilot heard a “significant boom” and noticed a reduction in engine rpm.

He unsuccessfully attempted to regain engine power and then initiated a forced landing to a space between trees near Wellston, Ohio.

During the forced landing, the airplane hit rolling, uneven grass terrain and a barbed wire fence.

Examination and disassembly of the engine revealed a large hole on the top of the engine crankcase inboard of the No. 4 cylinder. The examination revealed that the No. 4 connecting rod had failed due to the installation of a nonconforming small end connecting rod bushing.

Five days before the accident, the engine manufacturer had issued a mandatory service bulletin indicating that the accident connecting rod bushing may not have met engine specifications and may require follow-up action. Six days after the accident, the FAA issued an airworthiness directive that required inspecting the connecting rods, replacing affected connecting rod small end bushings, and accomplishing the instructions in the engine manufacturer’s mandatory service bulletin.

Probable cause: The failure of a connecting rod small end bushing, which resulted in a loss of engine power.

NTSB Identification: CEN17LA311

This August 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.